Almost all holes or bores that are drilled or machined into a workpiece are circular in cross section, for the obvious reason that a rotating drill or cutter describes a circle. There are specialty tools, such as a drill that cuts a hole of rectangular cross section, but these do not have wide application, since there is little call for bores of anything but circular cross section. There may, however, be a significant need in the future for a method to accurately and efficiently machine a bore of oval or elliptical cross section. Some promising research has indicated that an engine cylinder bore with such a shape might be a very cost effective way of packaging more combustion chamber volume within a given engine block size. In effect, the circular cross section of the cylinder bore is elongated, providing more bore volume without enlarging the block.
It is far more difficult to cut an elliptical surface than a circular one. While the bore can be cast initially to near net shape, it still has to be machined out to an accurate final shape. One known method consists of orbiting a cylindrical milling cutter around in the desired pattern inside the rough bore. Because such a tool has to be significantly smaller in diameter than the bore, but just as long, it is subject to flexing. Such a process is also slow. Broaching to final shape is also a possibility. Though accurate, a broaching process is inherently slow and expensive, and broaching a blind or closed bottomed hole, such as a cylinder bore, would be very difficult. Broaches are generally designed to be pushed or pulled all the way through the hole they are shaping.